The optical properties of a neuron change when its membrane potential is altered. Using these optical phenomena, we plan to construct an apparatus that will monotor action potentials and synaptic potentials in 100 neurons, individually and simultaneously. With such an apparatus we plan to determine the functional map of the connections between the cells of a simpler invertebrate ganglion. With further experiments it should be possible to determine in some detail the neuronal basis of the behaviour of the animal.